Originally Posted by
Chris W
"The byproducts in the exhaust of a running alcohol (glow) are water vapor and CO2. The exhaust residue on your glow powered plane is loaded with unburned fuel thus it is clear. The exhaust of gasoline or diesel are unburned Hydrocarbons and carbon and if a 2 cycle, engine oil. That's why the exhaust you see coming out of a glow engine is white clouds just like what you see in the sky (water vapor) whereas the gasoline or diesel is gray smoke. The oily residue in gasoline engines is a function of low oil content in the fuel. The black oily exhaust residue from the diesel is normal and is nothing more than carbon mixed with oil but not cut by unburned kerosene. The lighter the color say brown or golden brown in a very rich diesel thus diluting up the exhaust with unburned fuel."
http://www.rcuniverse.com/forum/ever...t-qustion.html
Well, that doesn't really answer much though.
In previous discussions here the general consensus has been that the color comes from temperature of the combustion. Larger engines (above .15) then tend to have a darker residue compared to smaller engines. A larger prop also means a lower load (and less developed power) and these then have less colouring of the exhaust residue compared to a smaller prop on the same engine.
A 9x4 prop on a .15 engine sounds about right, although one can use a little larger prop than that too (eg. 9x5 or 9x6).
So if this gives a very dark, black, exhaust residue then I would still say that the fuel could be the problem. I mainly use engines below .15 and when these are propped for around 10krpm at full throttle, the exhaust is only lightly coloured. It will get darker on smaller props and higher rpms but not black.
I don't have the MMVS D7 engine myself, was that produced at the same time as the Modela series?